Gust Electric Power - University Of Florida
Power and Electric EnergyPower and Electric EnergyzVoltage(Work) * Current(Charge) Power(Work)ChargeTimeTimezPower(Watts) Potential(Volts) * Current(Amperes) EIEnergy CalculationzWhat is the energy required to operate a 3000 Wheater for 20 minutes?¾¾¾Energy Power * TimeE 3000 W * 1200 sE 3,600,000 J1
ResistancezzThe relative difficulty with which current can betransmitted in a material is defined as theelectrical resistance of the material.Two quantities of voltage and current can berelated through the physical parameter,resistance. The voltage supplies the potentialforce in an electrical system. Flow of charge orcurrent is the desired result.Ohm’s LawzI E/R–––zWhere I current in amperes, AE potential in volts, VR resistance in ohms, ohmR ρ(L / A)––––Where R resistance, ohmL lengthA cross-sectional areaρ resistivityGustafson, Fundamentals in Electricity for Agriculture, 19882
Resistance of a ConductorzRt Ri (1 α T) ohm––––Where Rt resistance at specified temperature,ohmRi resistance at reference temperature, ohmα temperature coefficient of resistance, 1/oCT difference temperature between specified andoreference, CDirect and Alternating CurrentzzzElectrical systems are generally classed intotwo categories by the form of the current.Direct current (dc) is characterized by currentflow in only one direction at all times. Batteries,thermocouples, solar cells, and rotating dcgenerators are all examples of sources fordirect current systems.Alternating current (ac) is characterized byalternating flow in two directions.Gustafson, Fundamentals in Electricity for Agriculture, 19883
Direct and Alternating Current con’tzAn alternating current system is one in whichthe direction of flow changes periodically.¾e Em sin θ¾ Wheree instantaneous voltage maximum voltage¾ θ angle¾ EmAmplitude of Sine WaveszzzzOne of the most frequently measured characteristics ofthe sine wave is its amplitude.Peak voltage is the maximum amplitude of either thepositive or negative part of the cycle.The effective or rms (root mean square) of a sine waveis the value equivalent to the constant dc magnitudethat would provide the same amount of power.For example, many conventional residential wiringsystems operate at 120 V effective or rms. This meansthat a light bulb would glow at the same brightness on120 V rms AC or connected to 120 V DC source.Amplitude of Sine Waves con’tzErms EPeak / 20.5z Irms IPeak / 20.54
Phase Relations and Power in ACCircuitszWhen a sine wave voltage is imposed on a load, a sinewave current will result. For the case of a resistiveload, the voltage and the current waves are “in-phase”with each other. The term “in-phase” means thecurrent and the voltages go through zero and throughtheir peak values at the same time.Gustafson, Fundamentals in Electricity for Agriculture, 1988Power CurveGustafson, Fundamentals in Electricity for Agriculture, 1988zTrue or effective power as shown in the diagram isthe product of the rms voltage or current.Power Curve con’tzWhen a circuit contains elements with other thanpure resistance (capacitance or inductance), a phaseshift will occur between the voltage and currentwaves. That is, the waves will no longer cross zeroor reach peaks at the same time. The amount of shiftmeasured in degrees is called the phase-shift angleor the phase shift. This phase shift will affect theshape of the power curve.–Ex: motor circuit5
Power in AC CircuitGustafson, Fundamentals in Electricity for Agriculture, 1988zPower can be calculated by P EI cos φ–––zWhere E voltage (RMS), VI current (RMS), Aφ phase-shift angleNote that when the phase-shift angle is zero as in a purelyresistive circuit or a dc circuit, we return to the form¾Power Voltage * Current EI(lights, electric heaters, etc)Apparent PowerzApparent power is always greater than true power whenthe voltage and current are not in-phase.¾The ratio of true power to apparent power for a circuit isdefined as the power factor. The power factor is alsoequal to the cosine of the phase-shift angle. Power factorcan vary from a value of one, (in-phase, φ 0, cos 0 1), to a value of zero, (90 out of phase, cos 90 0).¾Power Factor cos φ true power wattsapparent powervolts*amperes¾Apparent Power E*I (VA)Power FactorzAn electrical system can be loaded to operatewith a power factor of 1.0. The system willdistribute energy at a maximum efficiencywhen the power factor equals one. However,this is not always economical or feasible in realsystems. Power generators are concerned withpower factor since it effects their line lossesand capacities.6
120/240 V Single-Phase ServiceSystemzzBy far the most common service system forfarms and residences in North America is the120/240 V three-wire single phase system. Thesystem originates at a step-down transformerfrom the local distribution system.The distribution system at a higher voltagefeeds the primary of the transformer and thesecondary side is the origin of the service dropto the user.Transformer for Origin of 120/240 VSystemGustafson, Fundamentals in Electricity for Agriculture, 1988120/240 V Single-Phase ServiceSystem con’tzIf a voltmeter is placed between the neutraland either hot wire, a voltage of 120 V will bemeasured. If the voltmeter is placed across thetwo hot conductors, a voltage of 240 V isobtained. The 240 V potential difference arisesfrom the addition of two 120 V ac sourceswhich are 180 electrical degrees out of phasewith each other.7
Voltage Waveforms for 120/240 VServiceGustafson, Fundamentals in Electricity for Agriculture, 1988Voltage for 120/240 V Service inPhasor FormGustafson, Fundamentals in Electricity for Agriculture, 1988120/240 V Single-Phase ServiceSystem con’tzzConsideration must be given as to how loadsare connected to this system and how theyeffect current level in each conductor.120 V loads may be connected between eitherHot conductor (commonly called legs) and theneutral. Loads at 240 V are connectedbetween the two Hot conductors. Note theneutral wire is not connected to the 240 Vloads. Therefore, the 240 V loads can notplace current on the neutral wire.8
120/240 V Load ConnectionsGustafson, Fundamentals in Electricity for Agriculture, 1988120/240 V Single-Phase ServiceSystem con’tzzLoads requiring 120 V potential will beconnected between the neutral and one of thehot conductors. In this case, current may becarried in the neutral wire.The amount of current flowing in the neutralcan be minimized by balancing the loads; thatis, by having equal 120 V loads connected toeach of the hot wires.120/240 V Single-Phase ServiceSystem con’tzzParticularly in livestock facilities, it is desirableto minimize the neutral current as much aspossible by the balancing of loads and use of240 V rather than 120 V equipment. Groundcurrents can stress animals.As a general rule, the neutral will carry only thenet difference or imbalance in current betweenthe 120 V loads on the two legs.9
Three-Phase SystemszWhere large quantities of electrical power arebeing transmitted or used three-phase acpower is generally used. Such currents aregenerated by an alternator having threeidentical armature coils spaced 120 degreesfrom each other. Since each coil has twoconnections, it at first appears that 6 lines areneeded to transfer currents to the loads.Three-Phase Systems con’tzzIt is possible to connect one end of each coil toanother coil at the alternator and then transmitthe current over 3 lines, one for each phase.Two basic considerations, called “wye” and“delta,” are used.Three-Phase DeltazzFor the delta configuration, the ends of each winding are joined tothe ends of the other 2 windings to get the characteristic triangle,or delta configuration.In this figure, the loads on the system are assumed to bebalanced. That is, all the loads are of the same magnitude.Gustafson, Fundamentals in Electricity for Agriculture, 198810
Three-Phase Delta con’tzzzzFor the three-phase system the terminology of phasevoltage, line-to-line, or phase-to-phase voltages, andphase and line currents are widely used.The voltage across one winding or a single load iscalled the phase voltage.The voltage between two of the conductors betweenthe source and the load is termed the line-to-linevoltage, phase-to-phase voltage, or simple line voltage.For the three-phase (3φ) delta, the phase voltage andline-to-line voltage are the same.Three-Phase Delta con’tzzzThe current through any winding or load segment iscalled the phase current.The current through one of the conductors from thesource to the load is termed the line current.In a delta configuration, a line current is the vector sumof two phase currents. Because of the 120o differencebetween each of the phase current, the vector sum ofany two, the line current, is equal to 30.5 (1.732) timesthe phase current.Three-Phase Delta con’tzzzzTo obtain single phase from adelta system, connections aremade across one of the threephases.A single-phase 240 V systemcan be obtained by connectingacross one phase of the threephase delta, as shown acrossphase B.If a single phase 120/240 Vsystem is needed, a neutral wireis connected to a center tap asshown in phase C.In practically all installations, theneutral wire would be grounded.Gustafson, Fundamentals in Electricity for Agriculture, 198811
Three-Phase WyezzA common application of the wye system isknown as the 3-phase, 4-wire system. Is has agrounded neutral connected to the commonjunction of the three transformers.In the 4-wire system, single-phase power canbe obtained by connecting between the neutraland any one of the phase conductors.Three-Phase, Four-Wire SystemGustafson, Fundamentals in Electricity for Agriculture, 1988Three-Phase Wye con’tzIf, for example, the three-phase voltage is 208V, a single phase at 120 V could be obtainedby connecting phase to neutral. Thus by using4 wires, we can have a 3-phase system at 208V for motors, water heaters, and similar largeloads and a single-phase system at 120 V forlighting and small appliances.12
Three-Phase PowerzzThe total power outputfor a three-phasesystem is a constantvalue.The total instantaneouspower, which is the sumof the three curves, is aconstant over time.Total true power outputfor the three-phasesystem is expressed by:Gustafson, Fundamentals in Electricity for Agriculture, 1988Power in a Three-Phase SystemGustafson, Fundamentals in Electricity for Agriculture, 1988Building Service EntranceszzBuilding service equipment are those components ofthe system needed to carry the current from the feederconductors to the branch circuits serving the loadswithin the building.The function of the service entrance is to supply powerto each of the branch circuits within the building fromthe service conductors. In doing so it must:––––Supply a main switch or disconnect meansSupply overcurrent protection for branch circuitsSupply terminals for attachment of branch circuit conductorsincluding equipment grounding conductorsSupply a point of connection to earth or ground the system.13
Single-Phase Service EntranceGustafson, Fundamentals in Electricity for Agriculture, 1988Electrical GroundingzzThe term grounding, in electrical terminology,means connected either directly or indirectly toearth. The purpose of grounding is for safety.Grounding can be divided into two sections;system grounding, which is grounding ofcurrent-carrying portions of the system andequipment grounding, which is grounding ofequipment not intended to be at a voltagepotential difference from the earth.System GroundingzzzThe principle reason for system grounding is to limitthe voltage between any conductor and the earth to aminimum value for the system being used.A ground can be a grounding electrode or anyconductive material connected directly or indirectly tothe earth. For example, metal pipes (water, gas, drain)in a building are connected to other pipes, which in turnare buried in the ground.Voltage to ground will be the voltage between anypoint in the electrical system and any object that isgrounded.14
System Grounding con’tzzThe typical 120/240 Vsingle-phase system hasthe neutral wiregrounded.If we measure from anygrounded object to eitherhot conductor, the voltagewill be 120 V. Thus thevoltage to ground is theminimum possible for a120/240 V system, andthe voltage from anyungrounded conductor toground is the same.Gustafson, Fundamentals in Electricity for Agriculture, 1988System Grounding con’tzzzHaving a minimum voltage and the same voltage toground for all ungrounded conductors are requirementsof the National Electric Code for safety.System grounding provides a measure of safety (a) forequipment or persons if an unintentional contactbetween a hot conductor and earth is made by limitingthe voltage to a minimum value or (b) if a fault outsidethe building should arise.Good system grounding is dependent on establishinglow resistance paths to earth at each system groundingpoint.System Grounding con’tzThe NEC (250-81) states that if available on the premises,at each building or structure, each of the following itemscould be bonded together to form the grounding electrodesystem:––––Metal underground water pipe with at least 10 ft (3 m) in the earthMetal frame of the building where effectively groundedConcrete-encased electrode – and electrode encased in at least 2in (51 mm) of concrete, located within or near the bottom of aconcrete foundationGround ring encircling the building at least 2.5 ft (76 cm) belowthe surface.15
System Grounding con’tzIf none of the electrodes specified above areavailable, one or more fabricated electrodes arerequired (NEC 250-83). The most commonelectrodes are a rod or pipe electrode. These rodsare to be not less than 8 ft (2.44 m) in length. If asingle electrode does not have a resistance toground of 25 ohms or less, one additionalelectrode must be added.Equipment GroundingzzzzThe NEC requires that all metal likely to become energizedby an electrical short be grounded to the same ground asthe system ground.To meet this requirement, equipment grounding supplies alow-resistance path from all of the metal objects to thesystem ground at the service entrance.In many instances, an extra conductor (green oruninsulated) will be used, or in others metal conduit maycreate the low resistance path.Equipment grounding is necessary to prevent electric shockto persons or animals coming in contact with metallicobjects, which due to some fault, have come in contact witha hot conductor.Equipment Grounding con’tzTo illustrate the potential problem, if a fault develops in themotor windings such that the hot conductor comes incontact with the ungrounded frame, a 120 V potential existsbetween the motor frame and earth.Gustafson, Fundamentals in Electricity for Agriculture, 198816
Equipment Grounding con’tzThis type of hazard can be avoided by equipmentgrounding the frame. If the hot conductor were to comein contact with the frame, a short circuit would exist,and the overcurrent protection for the circuit would then“blow” or open the circuit due to the high current flow.Gustafson, Fund. in Electricity for Agriculture, 1988Equipment Grounding con’tzIn the circuit the neutral conductor is grounded at the serviceentrance as is required by the NEC. The NEC also requiresthat the neutral conductor not be grounded at any other pointbeyond the service entrance. This is to assure that all loadcurrent returns via the insulated neutral conductor.Gustafson, Fundamentals in Electricity for Agriculture, 1988Equipment Grounding con’tzzIf the equipment grounding system is not properly installed andmaintained, it is possible that the overall resistance of the groundingpath will increase to the point where insufficient fault current will flow.Two problems include: obvious shock hazard and, if the conduit ismaking poor contact, there may be some arcing at the bad contact,which could ignite a fire. Even if arcing does not occur, danger mayexist from heating.Gustafson, Fundamentals in Electricity for Agriculture, 198817
Equipment Grounding con’tzUse of groundingelectrodes at remoteequipment can notsubstitute forequipment grounding. Ifa fault occurs to a pieceof equipment which isnot equipmentgrounded, the faultcurrent through thegrounding electrode willnot be sufficient toopen the overcurrentprotection and a hazardremains.Gustafson, Fundamentals in Electricity for Agriculture, 1988Polarity and SwitchingzzzIn most wiring systems, conventions have beenestablished for the color of the covering for eachconductor in the system.In a single-phase system, a white wire is always usedas the neutral, black or red are used as the “hot”conductors, and green or bare wires as the groundingconductors.Devices such as switches and outlets are alsopolarized by the use of brass colored terminals for hotcontacts, silver-colored terminals for neutral contacts,and green terminals for grounding contacts.Polarity and SwitchingzzzIf a circuit is correctly polarized, all switches will beplaced in the hot (red or black) conductors.When a switch is improperly placed in a neutralconductor a voltage exists between the electricalequipment and the ground even when the switch isopen.If for example, the device is a light bulb socket, thescrew shell would be a potential of 120 V. This couldmake changing the light bulb hazardous if the shell istouched.18
Polarity and Switching con’tzzIt should be apparent why the grounded neutralis never to be interrupted by a fuse, circuitbreaker, switch, or other device unless both thehot and neutral are switched together.Under no condition should switches or overcurrent protection be placed in a groundingconductor.Gustafson, Fundamentals in Electricity for Agriculture, 1988Overcurrent ProtectionzzzAny electrical system needs safeguards to assure thatsafe levels of current for conductors and equipment arenot exceeded. Two basic classes of devices are usedfor this purpose, fuses and circuit breakers.Both are installed in series with the hot conductors andare designed to open the circuit if a specified currentlevel is exceeded.Fuses are overcurrent devices of which a portion isdestroyed when interrupting the circuit. They are madewith low melting point links which are calibrated to meltwhen a specific current is reached.19
Overcurrent Protection con’tzzA standard fuse will carry a 10% overload for afew minutes, a 20% overload for less than aminute, and a 100% overload for only a fractionof a second.Often in circuits for control of motors, fuseswhich will allow a temporary overcurrent duringmotor starting are needed. Time-delay fusesare designed for this need.Plug-Type Time-Delay FuseGustafson, Fundamentals in Electricity for Agriculture, 1988Time-Delay FusezzzA time-delay fuse can open in either of two ways.During a continuous overload, a solder connection atone end of the link will melt, and the spring will pull thelink away, breaking the circuit.During a short circuit with a large current, the link itselfwill melt almost instantaneously.An overload created by a motor start will not be largeenough to melt the link or of long enough duration tomelt the solder connection.20
Time-Delay Fuse con’tzzThe most common fuses for residential wiring are the screw orplug type fuse. They have ratings up to 30A and are used withvoltages up to 125 V.When the circuit exceeds 30 A, it is necessary to use a cartridgetype fuse and fuse holder.Gustafson, Fundamentals in Electricity for Agriculture, 1988Circuit BreakerzzzA circuit breaker is a device designed to open a circuitautomatically at a predetermined overload currentwithout damage to itself. Most circuit breakers have abimetallic strip connected with the contacts.Current passes through the bimetallic strip causing it toheat up. Since the two metal expand at different rates,it also bends. If the current level is too high, the bendwill be large enough so that the contact points willopen.After the element has cooled, the circuit breaker can bereset.Circuit Breaker con’tGustafson, Fundamentals in Electricity for Agriculture, 198821
Ground Fault Circuit InterrupterszzzIn a complete electrical circuit there must be at leasttwo wires; one to carry the current to the load and oneto return the current to the outlet or source.If the insulation of the wiring or load is faulty or breaksdown, all or a portion of the current may follow analternate path through the grounding system or earthback to the source. This is called ground fault.A ground fault current may not be large enough tocause a fuse or circuit breaker to trip, but still be fatal toa person.Gustafson, Fundamentals in Electricity for Agriculture, 1988This table shows the current levels for human responseincluding levels at which a person can no longer let go ofa device and at which fibrillation occurs. These currentsare much lower than those required to activateovercurrent protection.Gustafson, Fundamentals in Electricity for Agriculture, 198822
ProtectionzzzzThree ways exist to attempt to protect persons fromground faults.One method is to use double-insulated tools whichinsulate the operator from the tool.The second is the use of three-wire (grounded) cordswhich extend the equipment grounding to the tool.However, all too often the third or grounded wire is notused, thereby rendering the ground wire useless.The third method is to use a device known as a groundfault interrupter.Ground Fault InterrupterzA ground fault interrupter measures the current in the twoload conductors. Whenever the difference exceeds aspecified value (like 5 mA) the device opens the circuit.Gustafson, Fundamentals in Electricity for Agriculture, 1988Ground Fault Interrupter con’tzzGround faults can occur almost anywhere but are mostserious in wet or damp areas. With this in mind, theNEC requires GFI’s in all circuits in new wiringsupplying receptacles in the bathroom and garages ofdwellings. They also require them on all outdoorreceptacles accessible from the ground level orassociated with swimming pools.Fuses and circuit breakers protect the system fromexcessively large currents while GFI’s protect personsfrom small leakage currents.23
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